5 - OOP - Smalltalk in a Nutshell (a)

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Stéphane DucasseStephane.Ducasse@univ-savoie.frhttp://www.iam.unibe.ch/~ducasse/

Smalltalk in a Nutshell

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Goals•Syntax in a Nutshell•OO Model in a Nutshell

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Smalltalk OO Model***Everything*** is an object

Only message passing Only late binding

Instance variables are private to the object and protected to the subclassesMethods are publicEverything is a pointer

Garbage collectorSingle inheritance between classesOnly message passing between objects

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Complete Syntax on a PostCard

exampleWithNumber: x “Illustrates every part of Smalltalk method syntax. It has unary, binary, and key word messages, declares arguments and temporaries, accesses a global variable (but not and instance variable), uses literals (array, character, symbol, string, integer, float), uses the pseudo variable true false, nil, self, and super, and has sequence, assignment, return and cascade. It has both zero argument and one argument blocks.”

|y|true & false not & (nil isNil) ifFalse: [self halt].y := self size + super size.#($a #a ‘a’ 1 1.0)

do: [:each | Transcript show: (each class name); show: (each printString); show: ‘ ‘].

^ x < y

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Language Constructs^ return

“ comments # symbol or array ‘ string [ ] block or byte array .separator and not terminator (or namespace access in VW) ;cascade (sending several messages to the same instance) |local or block variable := assignment $ character :end of selector name e, r number exponent or radix ! file element separator <primitive: ...> for VM primitive calls

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Syntax in a Nutshell (I)comment: “a comment”character: $c $h $a $r $a $c $t $e $r $s $# $@ string: ‘a nice string’ ‘lulu’ ‘l’’idiot’symbol: #mac #+ array: #(1 2 3 (1 3) $a 4)byte array: #[1 2 3]integer: 1, 2r101real: 1.5, 6.03e-34,4, 2.4e7float: 1/33boolean: true, falsepoint: 10@120

Note that @ is not an element of the syntax, but just a message sent to a number. This is the same for /, bitShift, ifTrue:, do: ...

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Syntax in a Nutshell (II)assigment: var := aValueblock: [:var ||tmp| expr...]

temporary variable: |tmp|block variable: :varunary message: receiver selectorbinary message: receiver selector argumentkeyword based: receiver keyword1: arg1 keyword2: arg2...cascade: message ; selector ...separator: message . messageresult: ^parenthesis: (...)

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Messages instead of a predefined syntax• In Java, C, C++, Ada constructs like >>, if, for,

etc. are hardcoded into the grammar• In Smalltalk there are just messages defined on

objects• (>>) bitShift: is just a message sent to numbers

– 10 bitShift: 2• (if) ifTrue: is just messages sent to a boolean

– (1> x) ifTrue: • (for) do:, to:do: are just messages to collections

or numbers– #(a b c d) do: [:each | Transcript show: each ; cr]– 1 to: 10 do: [:i | Transcript show: each printString; cr]

• Minimal parsing• Language is extensible

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How to Define a Class (Sq)?• Class Definition: A message sent to

another class

Object subclass: #TomagoshiinstanceVariableNames: ‘tummy

hunger dayCount’classVariableNames: ''poolDictionaries: ''category: ‘Monster Inc’

• Instance variables are instance-based protected (not visible by clients)

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Smalltalk defineClass: #Packet superclass: #{Object}

indexedType: #none private: false

instanceVariableNames: 'addressee originator contents'

classInstanceVariableNames: '' imports: '' category: 'LAN'

How to Define a Class (VW)?

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How to Define a Method?• Normally defined in a browser or (by

directly invoking the compiler)• Methods are public • Always return self

Tomagoshi>>digest "Digest slowly: every two cycles, remove one

from the tummy”

(dayCount isDivisibleBy: 2)ifTrue: [ tummy := tummy -1]

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How to Define a Method?Normally defined in a browser or (by directly invoking the compiler)Methods are public Always return self

Node>>accept: thePacket "If the packet is addressed to me, print it. Else just behave like a normal node"

(thePacket isAddressedTo: self) ifTrue: [self print: thePacket] ifFalse: [super accept: thePacket]

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Instance Creation1, ‘abc’

Basic class creation messages are new, new:, basicNew, basicNew:Monster new

Class specific message creation(messages sent to classes)

Tomagoshi withHunger: 10

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Messages and their Composition• Three kinds of messages

– Unary: Node new– Binary: 1 + 2, 3@4– Keywords: aTomagoshi eat: #cooky

furiously: true

• Message Priority• (Msg) > unary > binary > keywords• Same Level from left to right

• Example:• (10@0 extent: 10@100) bottomRight• s isNil ifTrue: [ self halt ]

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• Anonymous method• Passed as method argument or stored• Functions

fct(x)= x*x+3, fct(2). fct :=[:x| x * x + 3]. fct value: 2

Integer>>factorial tmp:= 1. 2 to: self do: [:i| tmp := tmp * i]

#(1 2 3) do: [:each | Transcript show: each printString ; cr]

Blocks

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SummaryObjects and MessagesThree kinds of messages

unarybinarykeywords

Block: a.k.a innerclass or closures or lambdaUnary>Binary>Keywords

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Roadmap•Syntax in a Nutshell•OO Model in a Nutshell

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Instance and ClassOnly one modelUniformly appliedClasses are objects too

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Lookup…Class + Inheritance

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Classes are objects too• Instance creation is just a message

send to a ... Class•Same method lookup than with any

other objects• a Class is the single instance of

amanonymous class• Point is the single instance of Point class

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Parallel Inheritance between classes

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Lookup and Class Methods

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Workstation withName: ‘BigMac’

aWorkstation name

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About the Buttons

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Summary- Everything is an object- One single model- Single inheritance- Public methods- Object based private attribute- Protected to subclasses- Classes are simply objects too- Class is instance of another class- One unique method lookup look in the class of the receiver